The present invention pertains to a method and apparatus for honing precision edges on a workpiece, such as a cutting tool, using an abrasive brush. The invention particularly relates to a process and apparatus for controlling the position of a cutting tool edge relative to an abrasive honing brush in order to provide precise controlled edge honing.
Cutting tools for cutting and shaping materials must be very hard to maintain their edges and withstand the high concentrated forces which are present at the cutting edge of the tool. These tools are frequently fabricated from carbide, ceramic, diamond coated carbide, CBN coated carbide or other tool materials which possess the necessary hardness. The disadvantage of using a hard material is that such materials tend to be brittle, and susceptible to crack formation. When cracks form, the material begins to chip, destroying the utility of the tool.
The predominant method of forming carbide edges on cutting tools uses a powder metallurgy process which involves placing powdered materials into a mold, and mechanically compacting them into specific tool geometric forms. The compacted tool form is then densified through a sintering process. The edges created by this process, however, are rough. Rough edges can adversely affect the performance of the tool, by increasing the tendency of the material to crack or chip. Furthermore, forces applied to the rough edge are not evenly distributed but, rather, are concentrated on high points of the edge. The low points of the edge tend to be sharp creating stress concentrations that increase the likelihood of crack formation. The rough edges on cutting tools can be smoothed by honing the edges before the tool is used in a machining process. Honing involves forming a rounded shape on the cutting edge of the tool. Early shapes were directed towards true radii, where the curvature of the smoothed edge was uniform across both surfaces adjacent to the edge.
More recently, edges having varying taper, i.e., non-uniform tapers about the periphery of the edge and generally called waterfall hones (see, FIG. 3c). Also, the correct sizing of the edge hone has been shown to affect tool life. As a result, the higher the precision with which the tool edges can be formed, the greater the resultant tool life.
Many different processes were originally used to smooth the edges of a cutting tool, including vibratory honing, mass media honing, slurry honing, honing inserts with media impregnated rubber wheels, dry blasting, wet blasting, and tumbling. These methods have several disadvantages, including intense labor requirements and poor predictability of edge hone characteristics between different tools exposed to the same honing process.
During the late 1970""s, a process of honing using a brush having bristles impregnated with abrasive media was developed. In this process, bristles are forced into contact with the edge of the cutting tool. The forced contact results in the removal of material along the edge. Brush honing the cutting tool edges has typically required high brush rotational speeds, resulting in the abrasive bristles striking the cutting tool edge, rather than being dragged across the edge.
In a conventional honing process, the brush is rotated such that the speed of the tips of the brush range from 3,000 to 12,000 feet per minute. In order for these conventional processes to be commercially feasible, a high speed has been necessary in order to hone a sufficient quantity of cutting tools in a short period of time.
The apparatus used in conventional honing processes require the placement of the cutting tools to be honed on a rotating table. As the table rotates, the part is translated along an arcuate path past a rotating abrasive brush. The rotating table allows a continuous honing process to be used, with cutting tools being loaded at one position, honed at a second position, and removed from the table at a third position. The individual cutting tools were rotated as they are passed through the stationary, rotating brush. The circular formation of the table also presents a compact area within which the honing process can be accomplished.
One drawback to the use of a rotary table to feed the cutting tool to the honing brush is that the arcuate path produces an uneven hone on the work piece.
More particularly, the arcuate path causes the contact between the tool edge and the honing brush to vary depending on the location of the tool on the path. As such, the resulting hone will vary across the edge of the part making precision honing very difficult.
Another deficiency with the prior methods of honing edges on the cutting tools is that the high bristle speeds result in the generation of excessive heat at the bristle tips. This heat causes the nylon bristles to partially melt, leading to nylon being deposited on the workpiece. The deposited nylon must then be removed before the tool can be coated, adding an additional step to the honing process.
Attempts have been made to cool the bristles by using fluid coolants to alleviate or reduce the build up of heat at the bristle tips. The coolant, however, creates a material disposal problem which is not desirable.
Also, conventional processes for honing tool edges do not typically permit variation of the rotational speed of the brush during the honing process. Instead, the speed of the table is normally controlled to vary the amount of material removed from the tool.
The present invention overcomes the disadvantages of the prior art by controlling the contact of the cutting tool edge with the bristles of the abrasive brush so that the cutting tool edge moves through the volume occupied by the bristles. Thus, the material removal action is distributed over a greater portion of the bristle, thereby reducing the build-up of heat in the bristles. The movement of the cutting tool edge into the volume of the bristles further results in a greater material removal rate due to the greater contact between the individual bristles and the cutting tool edge.
An apparatus is disclosed for honing at least one edge on a workpiece, such as a cutting tool. In one embodiment of the invention, the apparatus includes a base with a variable speed motor mounted on it. An abrasive brush is mounted to the motor and includes a plurality of bristles attached to a hub. The bristles each have a tip end and an interior end, with the interior end being fixed to the hub. The motor is adapted to cause the abrasive brush to rotate about an axis of rotation. The width of the abrasive brush is defined by first and second ends. The combination of the width of the brush and the length of the bristles defines a volume. The honing apparatus also includes a rotational controller means for controlling the rotational speed of the motor.
A mount for holding a workpiece is attached to the base. The mount includes a fixture for holding the workpiece, and a translational movement mechanism for controlling the position of an edge of the workpiece along a path substantially parallel to the axis of rotation of the abrasive brush.
In another embodiment, the motor is a fixed speed motor and the position of the workpiece edge relative to the abrasive brush is controlled by horizontal and vertical movement mechanisms.
A honing process is also disclosed for controlling the formation of a hone on the edge of a workpiece by controlling the movement and positioning of the workpiece through the volume of the rotating bristles. The movement and position of the workpiece is controlled so as to control the angle of impact between the bristles of the abrasive brush and an edge of the workpiece. The process results in the formation of precise tapered edges on the workpiece edge.